Natural gas found in deposits in the earth is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is usually transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by the gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. Unfortunately, this liquefaction contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels, such as gasoline and middle distillates, have been decreasing, and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Because those fuels are liquid under standard atmospheric conditions, they have the advantage that in addition to their value, they do not require the energy, equipment, and expense of the liquefaction process. Thus, they can be transported more easily in a pipeline than natural gas.
Therefore, for all of the above-described reasons, there has been an interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane (the major chemical component of natural gas) is reacted with oxygen to form synthesis gas (syngas), which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch (FT) process, carbon monoxide is reacted with hydrogen to form organic molecules known as hydrocarbons, which contain carbon and hydrogen atoms. Other organic molecules known as oxygenates, which contain oxygen in addition to carbon and hydrogen, also may be formed during the FT process.
The Fischer-Tropsch product stream commonly contains a range of hydrocarbons, including gases, liquids, and waxes. It is desirable to primarily obtain hydrocarbons that are liquids and waxes, e.g., C5+hydrocarbons, that may be processed to produce fuels. For example, the hydrocarbon liquids may be processed to yield gasoline, as well as heavier middle distillates. The hydrocarbon waxes may be subjected to additional processing steps for conversion to liquid hydrocarbons.
FT process is commonly facilitated by a catalyst having the function of increasing the rate of reaction without being consumed by the reaction. A feed containing syngas is contacted with the catalyst in a reaction zone that may include one or more reactors. Common catalysts for use in the FT process contain at least one catalytic metal from Groups 8, 9 , or 10 of the Periodic Table (based on the new IUPAC notation, which is used throughout the present specification). Cobalt metal is a particularly desirable catalytic metal in catalysts that are used to convert natural gas to heavy hydrocarbons suitable for the production of diesel fuel. Alternatively, iron, nickel, and ruthenium have served as the catalytic metal. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but the disadvantage of a high water-gas shift activity. Ruthenium has the advantage of high activity but is quite expensive.
The catalysts often further employ a promoter in conjunction with the principal catalytic metal. A promoter typically improves one or more measures of the performance of a catalyst, such as activity, stability, selectivity, reducibility, or regenerability. In addition to the catalytic metal, a FT catalyst often includes a support. The support is typically a porous material that provides mechanical support and a high surface area upon which the catalytic metal and any promoter are deposited.
The method of preparation of a catalyst may influence the performance of the catalyst in the FT reaction. In a common method of loading the catalytic metal to a support, the support is impregnated with a solution containing a dissolved metal-containing compound. When a promoter is used, an impregnation solution may further contain a promoter-containing compound. After drying the support, the resulting catalyst precursor is calcined, typically by heating in an oxidizing atmosphere, to decompose the metal-containing compound to a metal oxide. The preparation of the catalyst may include more than one impregnation, drying, and calcination cycles. When the catalytic metal is cobalt, the catalyst precursor is then typically reduced in hydrogen to convert the oxide compound to reduced “metallic” metal. When the catalyst includes a promoter, the reduction conditions may cause reduction of the promoter, or the promoter may remain as an oxide compound. As a result of the method described above, the catalyst precursor becomes an activated catalyst capable of facilitating the conversion of syngas to hydrocarbons having varying numbers of carbon atoms, and thus having a range of molecular weights.
Catalyst supports employed for the FT process have typically been refractory oxides (e.g., silica, alumina, titania, thoria, zirconia or mixtures thereof, such as silica-alumina). It has been asserted that the FT reaction is only weakly dependent on the chemical identity of the metal oxide support (see Iglesia, E. et al., Becker, E. R. et al. Ed. Computer-Aided Design of Catalysts., New York: Marcel Dekker, Inc., 1993.). Nevertheless, because it continues to be desirable to improve the activity of Fischer-Tropsch catalysts, other types of catalyst supports are being investigated.
In particular, various aluminum oxide compounds have been investigated. For example, gamma-alumina (γ-alumina) is an oxide compound of aluminum having, in its pure form, the empirical formula, γ-Al2O3. Gamma-alumina is distinguished from other polymorphic forms of alumina, such as alpha-alumina (α-Al2O3), by its structure, which may be detected, for example, by x-ray diffraction. As disclosed by B. K. Gan, et al. at http://www.us.iucr.org/iucr-top/cong/17/iucr/abstracts/abstracts/E0930.html, the structure of gamma-alumina is conventionally thought to approximate a spinel, with either a cubic or tetragonal symmetry. Gan, et al. further disclose that both cubic and tretragonal polymorphs may coexist.
In a common method of producing a gamma-alumina support, naturally occurring bauxite is transformed to gamma-alumina via intermediates. Bauxite is an ore that may be obtained from the earth's crust. Minerals commonly found in bauxite and the empirical formulas of their pure forms include gibbsite (α-Al2O3·3H2O), boehmite (α-Al2 O3.H2O), diaspore (β-Al2O3.H2O), hematite (α-Fe2O3), goethite (α-FeOOH), magnetite (Fe3O4), siderite (FeCO3), ilmenite (FeTiO3), anatase (TiO2), rutile (TiO2), brookite (TiO2), hallyosite (Al2O3.2SiO2.3H2O), kaolinite (Al2O3.2SO2.2H2O), and quartz (SiO2)
In a first transformation, gibbsite is derived from bauxite. The Bayer process is one common process for producing gibbsite from bauxite. The Bayer process was originally developed by Karl Joseph Bayer in 1888 and is the basis of most commercial processes for the production of gibbsite. As it is conventionally carried out, the Bayer process includes digestion of bauxite with sodium hydroxide in solution at elevated temperature and pressure to form sodium aluminate in solution, separation of insoluble impurities from the solution, and precipitation of gibbsite from the solution.
In a second transformation, boehmite is derived from gibbsite. As disclosed above, gibbsite is a trihydrated alumina having, in its pure form, the empirical formula αAl2O3.3H2O. Transformation of gibbsite to boehmite may be accomplished by varying the conditions so as to influence the thermodynamic equilibrium to favor boehmite. For example, a method for producing boehmite from gibbsite may include dehydratization in air at 180° C.
In a third transformation, gamma-alumina is derived from boehmite. Boehmite in its pure form is a monohydrated alumina having, in its pure form, the empirical formula α-Al2O3.H2O. Alternately, boehmite is denoted in the art by γ-AlO(OH). Boehmite is also sometimes called aluminum monohydroxide. The respective α and γ prefixes refer to the crystalline form. Boehmite is distinguished from other polymorphic forms of monohydrated alumina, such as diaspore (β-Al2O3.H2O), by its structure or crystalline form. In particular, boehmite typically has orthorhombic symmetry. Transformation of boehmite to gamma-alumina may be accomplished by varying the conditions so as to influence the thermodynamic equilibrium to favor gamma-alumina.
A support material for catalysts is desirably stable under reactive conditions. Under ambient conditions of temperature and pressure, such as for storage, gamma-alumina is less reactive and therefore more stable than boehmite. Thus, gamma-alumina is typically regarded as a more desirable support material than boehmite. Further, calcination of boehmite to form gamma-alumina before loading catalytic metal to the gamma-alumina is generally regarded as a desirable step in the formation of a catalyst from boehmite. Therefore, catalytic metals are not typically loaded to boehmite itself in forming a catalyst, but to more stable alumina phases such as gamma-alumina or another transition alumina.
The boehmite can be transformed to a gamma-alumina support via calcination, before loading the support with a catalytic metal such as cobalt. The calcination may be achieved, for example, by heating the boehmite in air to a temperature greater than the thermodynamic transformation temperature, which is about 500° C. at ambient pressure. The boehmite is usually calcined at a relatively high temperature of approximately 750° C. However, the surface area and overall volume of the support decreases as the calcination temperature increases, causing the metal surface area of the ensuing catalyst to be lower than desired.
It has been discovered that the catalyst has a higher hydrothermal stability when the boehmite is calcined in the presence of a catalytic metal precursor. The boehmite is impregnated with the catalytic metal precursor before calcination. The calcination proceeds at a temperature sufficient to decompose the catalytic metal precursor, desirably to an oxide of the catalytic metal. Further, the calcination proceeds at a temperature less than the temperature at which loss of support surface area is appreciable. Thus, when the catalytic metal includes cobalt, the calcination preferably proceeds at a temperature of at least 200° C. and less than about 800° C.
Unfortunately, the catalytic metal precursor migrates into the boehmite during the calcination, undesirably causing the size of the boehmite pores to change. Hence, this calcination method does not achieve the desired pore size on the catalyst support. As a result, the performance of the ensuing stabilized supported catalyst during the FT process is compromised. That is, syngas conversion and C5+hydrocarbon selectivity are not as high as desired. As such, a need exists to develop a process for making a catalyst from a boehmite material and a catalytic metal precursor without compromising the performance of said catalyst.